This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Objective

To investigate whether daily diet provides adequate selenium intake in healthy men
and women living in Tehran, Iran.

Method

Serum level of selenium was determined in 184 healthy individuals of both genders.
The samples were divided into two age groups, adults and children, for analysis. The
serum level of selenium was determined using hydride generation and flame atomic absorption
spectroscopy.

Results

The mean and standard deviation of serum selenium levels in children (1–16 years)
was 84.3 ± 11 μg/l and there was no significant difference between genders in this
group. In adults (older than 16 years) the mean serum selenium level was 100.6 ± 13
SD μg/l; among women the mean was 93.9 ± 14 SD μg/l and among men it was 102.2 ± 12
SD μg/l. The mean selenium level in men was higher than in women and data analysis
showed a significant difference between them (p < 0.005). There was also a positive
correlation between higher selenium serum concentration and age in men (P < 0.001).
Daily intake of selenium in men and women was calculated to be 67 μg and 62.1 μg respectively.

Conclusion

Our results show that the serum concentration of selenium in an Iranian population
is similar to other nationalities in the Middle East, particularly Saudi Arabia.

Keywords:

selenium; serum; Tehran inhabitants

Introduction

Selenium (Se) plays a key role in the maintenance of normal health in human populations
[1]. The cellular biochemistry of Selenium involves the expression of a variety of selenoproteins.
Selenium is part of the active site of glutathione peroxidase (GSH-Px), an antioxidant
enzyme [2]. It has been demonstrated that, when taken as a supplement, Selenium modulates the
cellular response to oxidative stress, inducing a faster restoration of the endogenous
antioxidative defense system against the production of reactive oxygen species [3]. Glutathion peroxidase controls the interacellular level of hydrogen peroxide, reducing
the formation of reactive oxygen species that can induce lipid peroxidations with
consequent damage to the cellular membranes [4]. Epidemiological studies suggest a low intake of Selenium might predispose an individual
to an increased incidence of cardiovascular disorders [5]. There is increasing evidence that selenium deficiency may have several serious short-
and long-term medical implications, including impaired immune response, or even cancer
[6]. An experimental study has shown that an increase in selenium level is associated
with decreased cancer mortality [7]. The recommended dietary allowance of Selenium in the USA is 55 μg/day for women
and 70 μg/day for men. In some regions of the world such as Finland, New Zealand,
the East coast of the United States of America and China the content of Selenium in
soil is remarkably low [4]. Therefore Selenium levels in the serum of populations throughout the world vary
from 41.7 μg/l in Finland to 158.2 μg/l in Canada [8]. There is currently no information of selenium intake and serum levels in the Iranian
population. The aim of our study was therefore to evaluate serum levels of Selenium
to find out whether daily diet provides adequate selenium intake to maintain the health
of men and women living in Tehran.

Materials and methods

1. Subject selection

Serum samples were collected from 184 random inhabitants of Tehran. An informed consent
was acquired, according to the guidelines from the Tehran university research ethic
committee. Some of the samples were collected from excessive serum residues of blood
taken from children who were referred for routine laboratory check up at the Children's
Medical Center. All the samples were tested to rule out HIV, HBV and HCV contamination.
Blood samples were left to coagulate spontaneously.

2. Determination of serum selenium

Blood samples (3 ml) were centrifuged at 3000 × g for 5 minutes. The clean serum was
stored at -70°C until the time of analysis. All glassware and bottles used for the
isolation of serum and for analysis were previously soaked in diluted nitric acid
(10%) and rinsed thoroughly with deionized water. This procedure was followed in order
to exclude the possibility of contamination with trace elements. Serum (500 μl) was
aliquoted into a vessel-tube for mineralization with 3 ml of HNO3/HCLO4 (4:1 v/v). The temperature of this mixture was slowly increased to 175°C until fumes
of HCLO4 appeared. The mixture was then heated according to the following (temperature/time)
scheme: 175°C/60 min, 200°C/60 min and finally 250°C for 60 min. The mixture was then
left to cool down to room temperature. HCL 6 N (10 ml) was added and heated to 170°C
for 30 min to reduce the Se (VI) to Se (IV). After cooling to room temperature, Se
concentration was determined using the hydride generation atomic absorption spectrometry
(Atomic absorption spectrometer Shimadzu, AA-680). Sodium borohydride solution (3
g NaBH4, 1 g NaOH in 100 ml of mili-Q water) was used as a reducing agent. All samples and
standards were analysed in duplicate. The accuracy of the procedure was evaluated
by analyzing commercially available samples of lyophilized human serum seronorm™ trace
element serum, level 1, M10181 indicating a recommended value of 81 μg/l, and seronorm™
trace element serum, level 2, NO0371 indicating a recommended value of 136 μg/l.

3. Statistical analysis

Kolmogorov-Smirnov tests were carried out for normal distribution. The reference range
for serum selenium was determined as the 95% confidence interval (CI) of means. Differences
in selenium concentration between the male and female populations were analyzed with
the Mann-Whitney U-test. P-values of less than 0.05 were considered significant.

Results

The studied individuals were all healthy, non-smokers. Volunteer medical history and
physical examination ruled out the presence of current disease in the studied individuals.
None of the individuals showed any digestive symptoms indicative of nutrient malabsorption.
The mean and standard deviation of the individual selenium levels in children (age
1–16) was 84.2 ± 11 μg/l, which was 85.1 ± 10.8 μg/l among females and 83.7 ± 11.2
μg/l among males. The mean serum selenium level in adults (over 16 years) was 100.6
± 12.9 μg/l, for adult women the mean was 93.9 ± 13.6 μg/l and for adult men was 102.1
± 12.3 μg/l (Table 1). The accuracy and precision of the methods used for selenium analysis are summarized
in Table 2.

Table 1. Age and sex of the studied individuals with corresponding selenium levels (μg/l)

Discussion

Selenium is an essential mineral in human nutrition. Natural selenium present in the
diet of humans is in the form of organic seleno-proteins such as selenomethionine
and seleno-cysteine. Foods such as fish and whole grain cereals are especially rich
in organic selenium compounds [11]. Selenium in cereals is primarily in the form of selenomethionine. This naturally
occurring amino acid is the most important nutritional form of selenium.

Deficiencies of selenium contribute to the prevalence and severity of iodine deficiency
disorders which are the most important and well-known global nutritional problems,
primarily in less developed countries [12]. Iodine deficiency in childhood impairs neuromotor and intellectual development,
with an average reduction in the intelligence quotient of 10 points [13]. Selenium is required in thyroid metabolism, converting inactive thyroid hormone
into active thyroid hormone [14]. It has been shown that in goitrous children who are both Se and iodine deficient,
major Se deficiency partially blunts thyroid response to iodine supplementation [15]. The mean serum Se level for healthy children (age 1–16) observed in this study was
84.2 ± 11 μg/l with no significant difference between sexes (p = 0.659). Table 3 shows the mean serum Se level of Iranian children compared to children from different
countries.

Table 3. Comparison of the mean serum levels of selenium, among children from different countries
(as mentioned previously [10])

The mean serum Se level for adults observed in this study was 100.6 ± 12.8 μg/l, which
was similar to the one reported in a survey in Saudi Arabia [16]. In the Nutritional Prevention of Cancer (NPC) Trial [17], a Selenium level of 80 ng/mL is considered the minimum level of plasma selenium
necessary in the bloodstream for maximum production of selenoproteins (glutathione
peroxidases, thioredoxin reductase, etc.).

Our results show that in adults there is a significant difference between men and
women (p < 0.005) with a higher concentration of selenium in men. This suggests a
sex-linked hormonal influence over serum level of selenium. It has previously been
shown that selenium is essential for spermatogenesis [18]. This trace element is present in the protein of the capsule surrounding the sperm
mitochondria and may have a structural function [19]. Our data also show a positive correlation between a higher concentration of selenium
in serum and age in men (P < 0.001).

Table 4 summaries the selenium serum levels in the Iranian population in comparison with
different countries. It is higher than levels calculated for Finland and other countries
where soil is poor in selenium content. By using the medium correlation factor (1.51)
as introduced by Navarro et al., to estimate the daily intake of nutrients, the daily
intake of Se was calculated as 62.19 μg in the female and 67 μg in the male populations
[20].

Table 4. Comparison of the mean serum levels of selenium, among adults from different countries
(as mentioned previously [9, 11])

Considering the American RDA, which recommends a daily Se intake of 50 μg for women
and 70 μg for men, it seems that the normal Iranian diet has an adequate content of
selenium for both genders.

Tehran, the capital of Iran, is located in the north of the country and has a population
of approximately fifteen million people, representing a large proportion of the country's
total population, estimated to be seventy five million people. The diversity of nourishment
sources, regional variation and different ethnic diets makes it difficult to extend
these results to the whole population.

Acknowledgements

This study was supported in part by a grant from the Tehran University of Medical
Sciences. The authors would like to acknowledge Dr. Verity A. Cadd at Oxford University
for kind revision of the manuscript.